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Excitable Cells - Lecture 6 (15/02/2018)
The Action Potential
There are ion concentration gradients across a cell membrane.
Not all ions can cross membrane at rest.
Potassium channels have the greatest permeability.
If potassium ions try to leave the cell, a negative charge is created.
This negative charge opposes further movement of potassium out of the cell.
Hardly any ions have to move to create a charge, so ion concentrations don't really change much.
Most of the charge lines up either side of the membrane creating a capacitor/resting potential.
We can calculate the amount of negative charge needed to exactly balance out the concentration gradient (the Equilibrium Potential), via the Nernst equation to do this.
Because potassium is much more permeable than all the other ions, the Resting membrane potential is close to the EK.
There is a small, but finite permeability to Na+ which means that the means Resting membrane potential is slightly more positive than the EK.
Key to communication in the nervous system:
Cochlear hair cells (hearing).
Excitable cells allow sensing of the environment and response to it.
Most (but not all) excitable cells use action potentials in order to generate a response.
The Action Potential
An Action Potential is the transient reversal of the membrane potential, from an inside-negative resting potential to insidepositive.
The duration of an Action Potential can vary from a few milliseconds in the nerve and skeletal muscle to a few hundred milliseconds, such as in the heart.
The responses of Action Potentials are all or nothing.
A larger stimulus is the fixed size of an action potential.
Small (sub threshold) stimuli cause no action potentials at all.
Once the threshold has been reached it doesn't matter how large the stimulus us, as the action potential has a fixed size.
Instead the body codes the stimulus intensity through changes in frequency, not the size of the amplitude.
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